Mixing is an important method to improve the performance of surfactants due to their synergistic effect. The changes in bonding interaction and adsorption structure of IM and OP molecules before and after co-adsorbed on Fe(001) surface is calculated by DFTB+ method. It is found that mixture enable the inhibitor molecules with higher EHOMO donate more electrons while the inhibitor molecules with lower ELUMO accept more electrons, which strengthens the bonding interaction of both inhibitor agent and inhibitor additive with metal surface. Meanwhile, water molecules in the compact layer of double electric layer are repulsed and the charge transfer resistance during the corrosion process increases. Accordingly, the correlation between the frontier orbital (EHOMO and ELUMO of inhibitor molecules and the Fermi level of metal) and inhibition efficiency is determined. Finally, we propose a frontier orbital matching principle for the synergistic effect of inhibitors, which is verified by electrochemical experiments. This frontier orbital matching principle provides an effective quantum chemistry calculation method for the optimal selection of inhibitor mixture.

Mentions:
Frontier orbitals of surfactant molecules usually locate at their active groups26. Figure 1 shows the distribution of HOMO and LUMO orbitals of OP and IM molecules, which is obtained by DMOL3 calculation. It is seen that the HOMO orbital of OP molecule locates at its benzene ring and the oxygen atom of polyoxyethylene chain attached to the benzene ring, while the LUMO orbital mainly locates at the benzene ring. This suggests that when OP molecule adsorbs on metal surface, the benzene ring of OP molecule can obtain and donate electrons while oxygen atom is mainly inclined to donate electrons. The HOMO orbital of IM molecule mainly locates at its benzene ring, while the LUMO orbital mainly locates at imidazoline ring. That means that the benzene ring of IM molecule can donate electrons whereas its imidazoline ring can obtain electrons. These active groups can adsorb on Fe surface through electron transfer.

Mentions:
Frontier orbitals of surfactant molecules usually locate at their active groups26. Figure 1 shows the distribution of HOMO and LUMO orbitals of OP and IM molecules, which is obtained by DMOL3 calculation. It is seen that the HOMO orbital of OP molecule locates at its benzene ring and the oxygen atom of polyoxyethylene chain attached to the benzene ring, while the LUMO orbital mainly locates at the benzene ring. This suggests that when OP molecule adsorbs on metal surface, the benzene ring of OP molecule can obtain and donate electrons while oxygen atom is mainly inclined to donate electrons. The HOMO orbital of IM molecule mainly locates at its benzene ring, while the LUMO orbital mainly locates at imidazoline ring. That means that the benzene ring of IM molecule can donate electrons whereas its imidazoline ring can obtain electrons. These active groups can adsorb on Fe surface through electron transfer.

Mixing is an important method to improve the performance of surfactants due to their synergistic effect. The changes in bonding interaction and adsorption structure of IM and OP molecules before and after co-adsorbed on Fe(001) surface is calculated by DFTB+ method. It is found that mixture enable the inhibitor molecules with higher EHOMO donate more electrons while the inhibitor molecules with lower ELUMO accept more electrons, which strengthens the bonding interaction of both inhibitor agent and inhibitor additive with metal surface. Meanwhile, water molecules in the compact layer of double electric layer are repulsed and the charge transfer resistance during the corrosion process increases. Accordingly, the correlation between the frontier orbital (EHOMO and ELUMO of inhibitor molecules and the Fermi level of metal) and inhibition efficiency is determined. Finally, we propose a frontier orbital matching principle for the synergistic effect of inhibitors, which is verified by electrochemical experiments. This frontier orbital matching principle provides an effective quantum chemistry calculation method for the optimal selection of inhibitor mixture.